1. Field of the Invention
This invention relates to a disk, such as an optical disk, that is applied to a magnet clamp type recording and/or reproducing apparatus and, more particularly, to a disk having a-disk hub at the center thereof adapted to be attracted to and supported by a magnet of a disk drive device.
2. Description of the Prior Art
Up to now, there is known a disk recording and/or reproducing device in which a disk such as an optical disk is employed as the recording medium for information signals.
In the recording and/or reproducing apparatus in which the disk is used as the recording medium, the disk is revolved at an elevated speed of several hundreds to one thousand and several hundreds r.p.m. while a laser beam is irradiated on the recording surface of the disk for recording and/or reproduction. In order to assure a high speed rotation of the disk, clamp means are provided whereby the disk can be integrally connected to the disk drive unit and revolved at an elevated speed. As means for clamping the disk to the disk drive unit, so-called magnet clamp type clamp means has been proposed in which a magnet is provided to the disk drive unit and a disk hub having a magnetic metal plate is provided at the center of the disk. The metal plate is magnetically attracted by the magnet so that the disk is integrally retained by the disk drive device.
It is noted that, in the case of the disk applied to the magnet clamp type disk drive device, it is necessary that a clamping metal plate be attached to a disk hub mounted to the center of the disk as described above. Therefore, the disk is constituted by a supporting member secured to the disk substrate and a clamping magnetic metal plate.
However, since the supporting member must be secured to the disk substrate, it is formed of synthetic material different in material quality from the metal plate. Hence, the supporting member and the metal plate have different heat contraction coefficients. Therefore, when the metal plate is integrally mounted to the supporting member by metal mold casting, such as outsert or insert casting, and when the disk hub is secured as by ultrasonic welding to the center of the disk substrate of the disk, excess stress may be produced in the disk substrate at the time of welding or temperature rise in the disk substrate due to the aforementioned differential heat contraction between the supporting member and the metal plate, thus occasionally resulting in distortion of the disk substrate surface. Such excess stress on the disk substrate or distortion in the disk substrate surface tends to cause double refraction of the laser beam incident on and reflected from the disk substrate at the time of disk recording and/or reproduction, thus occasionally causing reading and writing errors of information signals.
In order to prevent unnecessary or excess stress of the disk substrate and distortion of the disk surface and the resulting double refraction of the laser beam as described above, it is also known to construct the disk hub of two separate members, that is, a supporting member and a metal plate, and to have the metal plate mountable to the supporting member.
FIG. 1 shows a disk applied to this magnet clamp type disk drive device.
In the optical disk shown in FIG. 1, a disk hub 52 for attachment to the disk drive device is mounted to the center of a disk substrate 51 having a central opening 50. The disk hub 52 is comprised of a ring-like member 55 secured to the center of the disk substrate 51 and having a spindle hole 54 engaged by a center spindle 53 of the disk drive unit adapted for rotationally driving the disk, and a clamping metallic plate 56 mounted on the ring-like member 55. Retaining pieces 57 are provided to the metal plate 56. These retaining pieces 57 are formed by cutting a portion of the surface of the metal plate 56 facing the magnet of the disk drive in the form of a letter U by punching, and bending the inner piece towards the lower surface. In the metal plate housing section 58, retaining apertures 59 for engaging and retaining the pieces 57 are formed at positions in register with the retaining pieces 57.
In this manner, by introducing the retaining pieces 57 into the registering retaining apertures 59 in the ring-like member 55 and turning the metal plate 56 in the retaining direction, the metal plate 56 can be integrally engaged with and retained by the ring-like member 55.
The above described optical disk is mounted to the disk table 60, as shown in FIG. 2, with the center spindle 53 engaged in the spindle hole 54 of the ring-like supporting member 55 of the disk hub 52. On the disk table 60 and at positions registering with the metal plate 56 in the disk hub 51, a magnet 61 is positioned with a small gap g from the metal plate. The metal plate 56 mounted on the disk hub 51 is attracted magnetically so that the optical disk may be revolved in unison with the rotation of the center spindle 53 of the disk drive unit. It is noted that the small gap g is provided between the metal plate 56 and the magnet 61 because it would be difficult to take out the disk from the disk table 60 if the metal plate 56 was completely attracted by the magnet 61.
It is noted that, when attaching the optical disk to the disk drive unit, the non-record portion on the inner peripheral side of the disk substrate 51 abuts on a reference mounting surface 62 of the disk table 60 in order to determine the mounting position of the optical disk.
It is noted that, when driving the disk into rotation by the disk drive unit, the speed and the amount of rotation of the disk cannot be controlled unless the rotation of the center spindle and the optical disk in unison with each other is assured. For this reason, the disk need be sufficiently magnetically attracted by the magnet so as to be made fast with it, in order that the optical disk will not slip on the disk table. However, an excessively strong magnetic attraction may result in the optical disk being unable to be detached easily from the disk table. Occasionally, the disk hub may be disengaged from the disk substrate due to the force of attraction of the magnet such that only the disk substrate is detached.
Therefore, the force of magnetic attraction of the magnet should be such as to prevent the disk slip and to enable the disk to be detached easily from the disk table. It is therefore necessary to select the force of magnetic attraction of the magnet and to maintain sufficient positional accuracy of the metal plate in the disk hub with respect to the magnet so that the aforementioned gap may always have a predetermined constant value.
However, when the retaining pieces are provided to the metal plate by punching or bending and these retaining pieces are engaged in the retaining holes of the disk hub for securing the metal plate in the disk hub, the retaining pieces are formed by bending a part of the metal plate, so that fluctuations are caused in the bend angle due to socalled springback and accuracy in the bend of the retaining portions. Hence it becomes difficult to maintain the height of the metal plate with respect to the disk substrate forming the mounting reference plane within the allowable dimensional tolerance so that the accuracy in the position of the metal plate cannot be assured sufficiently. In addition, due to the fluctuations in the bend angle of the retaining pieces of the metal plate, the metal plate cannot be mounted parallel to the disk substrate, resulting in the fluctuations in the extent of the gap g. Also, as a result of repetition of the mounting and dismounting of the disk to and from the disk drive device, the metal plate may be detached under the force of attraction by the magnet, or the disk hub may be peeled from the disk substrate under the load or unusual force due to the oblique mounting position of the metal plate.
In addition, the retaining pieces of the metal plate are formed by punching a portion of the surface of the metal plate that faces the magnet, these retaining pieces being retained by the retaining apertures in the disk hub, so that the metal plate and the magnet cannot be confronted to each other resulting in the lowered force of magnetic attraction.